The field of the present invention is aerogel composite materials. More particularly, this invention is directed to aerogel composites wherein the resulting composite exhibits improved performance as compared to prior aerogel composite products in one or more of the following qualities: reduced aerogel sintering; higher temperature performance; improved flexibility and drapeability; improved durability; decreased aerogel particle shedding; enhanced x-y plane thermal conductivity; enhanced x-y plane electrical conductivity; enhanced radio frequency interference (RFI) and/or electromagnetic interference (EMI) attenuation, enhanced infrared radiation (IR) suppression; and/or enhanced burn-through resistance. The fiber reinforcement is preferably a combination of a lofty fibrous structure (batting), individual randomly oriented short microfibers, and conductive layers. More particularly both fiber reinforcements are based upon either organic (e.g. thermoplastic polyester) or refractory (e.g. silica) fibers.
Insulating materials have been developed to solve a number of physical problems. Stiff polymeric foam and fiberglass insulating boards are well known as insulators for low and high temperature applications in fields such as refrigeration, building construction, and heating systems. Flexible battings such as those made from fiberglass have been used in applications that required flexibility, low density, and the ability to expand to fill a void space such as building construction. Aerogels, more specifically aerogel composites, were developed seeking to combine the strengths of both classes of materials.
Aerogels describe a class of material based upon their structure, namely low density, open cell structures, large surface areas (often 900 m2/g or higher) and sub-nanometer scale pore sizes. Supercritical and subcritical fluid extraction technologies are commonly used to extract the fluid from the fragile cells of the material. A variety of different aerogel compositions are known and may be inorganic or organic. Inorganic aerogels are generally based upon metal alkoxides and include materials such as silica, carbides, and alumina. Organic aerogels include carbon aerogels and polymeric aerogels such as polyimides.
Low density aerogels (0.02–0.2 g/cc) based upon silica are excellent insulators, better than the best rigid foams with thermal conductivities of 10 mW/m-K and below at 100° F. and atmospheric pressure. Aerogels function as thermal insulators primarily by minimizing conduction (low density, tortuous path for heat transfer through the nanostructures), convection (very small pore sizes minimize convection), and radiation (IR suppressing dopants may easily be dispersed throughout the aerogel matrix). Depending on the formulation, they can function well at temperatures of 550° C. and above. However, in a monolithic state they tend to be fragile and brittle and are thus not well suited for most applications outside of the laboratory.
U.S. Pat. No. 5,306,555 (Ramamurthi et al.) discloses an aerogel matrix composite of a bulk aerogel with fibers dispersed within the bulk aerogel and a method for preparing the aerogel matrix composite. The fibers may be long or short fibers of varying thicknesses, whiskers, mineral wool, glass wool, and even particles. The composition of the reinforcing material is an oxide such as SiO2 and Al2O3 (fibers, whiskers, and wools) and carbon, metals, and a variety of oxides (particles). Preferred fibers are glass wool and rock wool. The fibers may be randomly distributed or oriented. They may also be in the form of individual fibers, bundles of fibers, mats or sheets, woven or unwoven. The aerogel matrix composite is substantially crack-free with substantially no volume shrinkage. The composites are formed by infiltrating fibrous pre-forms, either woven or non-woven, with gel precursors, followed by drying of the wet gel under supercritical conditions. The products can be obtained on the scale of about 3–7 hours, but suffer a major drawback of having a high elastic modulus, making the products quite stiff as manufactured. The Ramamurthi et al. articles improve in flexibility as they are utilized because they form cracks in the aerogel matrix domains. A second drawback is that the thermal conductivities of the aerogel matrix composites are also relatively high (18 to 21 mW/m-K at ambient conditions) compared to the preferred embodiments of this invention 8.6 to 14 mW/m-K at ambient conditions).
U.S. Pat. No. 5,789,075 (Frank et al.) appears to describe the same structure as Ramamurthi et al. after the Ramamurthi et al. structure is removed from its mold, except that the Frank et al. composite is intentionally cracked in a controlled manner. The controlled cracking is said to give additional flexibility to the resulting composite. Suitable fibers are individual fibers randomly or ordered, preferably at least 1 cm in length. The fibers may also be used in the form of a web or mat. A plurality of webs or mats can be superposed upon one another. In the case of a layered arrangement of mats, a change in the direction from one layer to the next is deemed advantageous. Although the Description and Claims disclose a manufacturing process which includes step (b) “adding fibers to the sol,” the Examples only show the addition of a non-fiber-containing sol to a polyester or glass fiber web. Individual randomly distributed fibers are not used in combination with a fibrous web.
U.S. Pat. No. 5,972,254 (Sander) is directed to ultra-thin pre-stressed fiber reinforced aerogel honeycomb catalyst monoliths. Thin panels or monoliths of aerogels, xerogels, zeolites, and other low density material are reinforced with pre-stressed fibers in two of three dimensions. A mixture of metal alkoxides, water, and a catalyst are poured into a gas permeable mold containing pre-tensioned reinforcing fibers running perpendicular to each other at defined intervals, followed by polymerization and supercritical drying.
U.S. Pat. Nos. 5,973,015 and 6,087,407 (Coronado, et al.) describe aerogel composites made from organic precursors, e.g. formaldehyde, which infiltrate a fiber pre-form. The resultant composite is said to have mechanical stability. The re-inforcing fibers described in the figures run lengthwise and are shown to be planar structures in the figures. The products suffer from relatively low thermal stability in air under high heat loads as well as insufficient flexibility for many uses.
U.S. Pat. No. 6,068,882 (Ryu et al.) disclose aerogel composite materials previously manufactured and sold by Aspen Systems, Inc. The aerogel contents of the product were an aerogel powder rather than an aerogel monolith. Thus flexure of the product resulted in the shedding of significant quantities of the powder. The thermal performance was significantly degraded as compared to aerogel monolith alone. The prior products were stiff and readily fractured or fragmented.
Thus the prior aerogel composite materials have not been suitable for many uses due to one or more of: low flexibility, low durability, excessive aerogel sintering when exposed to heat, less than ideal thermal conductivity, insufficient x-y thermal and/or electrical conductivity, poor RFI-EMI attenuation, and/or insufficient burn-through resistance.
The present invention arose from research directed to resolving these problems. Accordingly, it is an object of the present invention to produce an improved aerogel composite structure which exhibits one or more of the following qualities: low sintering/higher temperature performance; improved flexibility, exceptionally low thermal conductivity, drapeability, or conformability; enhanced x-y thermal and/or electrical conductivity; enhanced RFI-EMI attenuation; and/or enhanced burn-through resistance.